The present invention relates to a composition containing poly(lactic acid), at least one bacteriocin (e.g., nisin, generally in the form of Nisaplin®), and at least one plasticizer (e.g., lactic acid, lactide, triacetin, glycerol triacetate), and optionally at least one pore forming agent. The present invention also relates to a method of making the composition, involving mixing about 100% of the total of the poly(lactic acid), about 50% to about 90% of the total of the at least one plasticizer, and optionally at least one pore forming agent at a first temperature of about 150° to about 170° C. to form a mixture, cooling the mixture to a second temperature of about 115° to about 125° C., adding at least one bacteriocin and about 10% to about 50% of the total of the at least one plasticizer and the remainder of the total of the poly(lactic acid) to the mixture and mixing to form the composition.
Nisin is a short chain antimicrobial polypeptide consisting of 34 amino acids. It is obtained from the culture of the food grade lactic acid bacteria Lactoccocus lactis subsp. Lactis. Nisin has demonstrated antimicrobial activity against a wide range of Gram-positive spoilage and pathogenic bacteria, and is the only bacteriocin approved for applications in food by the FDA (Cleveland, J., et al., Int. J. Food Microbiol., 71:1-20 (2001); Nisin preparation: affirmation of GAS status as a direct human food ingredient, Federal Register, 21 CFR Part 84, 1988, pp. 11241-011251). Currently, nisin is commercially available in a formulation containing 97.5% of milk solids and salts with the trade name Nisaplin®. Nisin, generally in the form of Nisaplin®, is popularly used in foods and beverages that are pasteurized but not fully sterilized, such as cheese, milk, and desserts. It is also used as a food preservative for meat and seafood (Calo-Mata, P., et al., Current applications and future trends of lactic acid bacteria and their bacteriocins for the biopreservation of aquatic food products, Food Bioprocess Technology, Springer, N.Y., 2007, Vol. 1, pp 43-63; Delves-Broughton, J., Food Australia, 57: 525-520 (2005); Stoyanova, L. G., et al., Applied Biochemistry and Microbiology, 43: 604-610 (2007); Sanjurio, K., et al., Food Research International, 39: 749-754 (2006)).
Nisaplin® can be applied to food by direct mixing with the foods, or by dipping the foods in Nisaplin® solutions. These methods may result in a rapid, on-site and on-time reduction of the bacterial population; however, these methods require a large amount of Nisaplin® and can not prevent the recovery of bacterial growth due to the short life time of Nisaplin® in foods. Alternatively, Nisaplin® can be incorporated into polymeric films that serve as food packaging which maintain food safety and quality, and prolong the shelf life of packaged foods. A variety of biobased materials has been used for this purpose, including chitosan, alginate, casein, cellulose derivatives, soybean proteins, zein, and animal derived proteins (Cha, D. S., et al., Lebens Wisse Technology, 35: 715-9 (2002); Kristo, E., et al., Food Hydrocolloids, 22: 373-86 (2008); Li, B., et al., Carbohydr. Polym., 65: 488-94 (2006); Millette, M., et al., Food Control, 18: 878-84 (2007); Xu, X., et al., Carbohydr. Polym., 70: 192-7 (2007)). Incorporation of Nisaplin® into petroleum derived thermoplastics such as poly(vinyl chloride) and low density polyethylene has also been studied in attempts to obtain antimicrobial films with higher tensile strength (Ming, X., et al., J. Food Sci., 62: 413-415 (1997); Siragusa, G. R., et al., Food Microbiology, 16: 229-235 (1999)).
Poly(lactic acid) (PLA) is a biodegradable thermoplastic produced from biobased precursors. PLA is easily processable and water resistant. Thin PLA membranes are good water vapor barriers and have relatively low gas transmittance, showing a high potential for packaging applications (Cutter C. N., Meat Science, 74: 131-142 (2006); Suyama, N. E., et al., J. Polym. and Env., 12: 1-6 (2004)). As previously reported, Nisaplin® could be incorporated into PLA by methods of diffusion, or by co-extrusion with the polyester following complexation with pectin, or by mechanical mixing in a PLA/CH2Cl2 solution, followed by film casting (Liu, L. S., et al., J. Appl. Polym. Sci., 106: 801-810 (2007); Jin, T., and H. Zhang, J. Food Sci., 73: M127-134 (2008)). Resultant PLA/Nisaplin® composites were antimicrobial. However, the processing was somewhat complicated and a large volume of organic solvent was also used in some cases. An ideal approach to prepare antimicrobial PLA membranes incorporating Nisaplin® is to co-extrude the two in one step, which would be simple, efficient, and could be easily handled for quality control and quality assurance. However, PLA melts at around 160° C.; while the maximal temperature at which nisin can retain its bioactivity is 120° C. Therefore, while nisin/PLA films could be prepared using one-step extrusion, the required temperatures for melting of the PLA during the process would result in films with little or no antimicrobial activity.
Thus there remains a need to produce PLA/bacteriocin (e.g., nisin) films at temperatures where the resulting films retain most or all of the antimicrobial activity of the bacteriocin.